William J. Bratton was named police commissioner of New York City for the second time on Thursday. But it is a different place than the crime-ravaged city he came to in 1994. And he said he was going to be a different kind of commissioner, overseeing a different kind of policing.

“In this city, I want every New Yorker to talk about ‘their police’, ‘my police,’ ” Mr. Bratton said after his appointment was announced by Mayor-elect Bill de Blasio, before reading from a children’s book about police work he said he had cherished since he was 9.

In 1994, the message was different: “We will fight for every house in the city; we will fight for every street; we will fight for every borough,” he said at the time. “And we will win.”

Back then, the hard-driving, press-savvy Mr. Bratton could be found dining out among city luminaries, and on the covers of newspapers and national magazines. He received a lot of credit for historic drops in crime rates, even as the trends in New York coincided with those around the country. Such prominence drove a very public wedge between him and Mayor Rudolph W. Giuliani, a former prosecutor, who pushed him out just two years after appointing him. The mayor has the full authority to hire and fire the commissioner.

On Thursday, Mr. Bratton, 66, said he had “learned a lot” since the last time he ran the New York department, the largest in the country, and people familiar with his thinking suggested that significant changes were in store.

Before making the decision to bring him back, Mr. de Blasio said he had been reassured by conversations with others who had worked with Mr. Bratton, especially in Los Angeles, where he lead the police from 2002 to 2009.

Mr. Bratton said: “I report to the mayor. I am not the mayor.”

But his is perhaps the single most important and visible appointment to be made by Mr. de Blasio, who was elected partially because of his criticism of the police’s aggressive use of the stop-and-frisk tactics. Mr. Bratton has been in law enforcement for four decades, including stints as commissioner in Boston as well as Los Angeles, but this may be his biggest challenge: keeping crime at historic lows — just more than 300 murders so far this year — while mending the relationship with minorities, and not upstaging his boss.

A bright mood reigned at a community court in Red Hook, Brooklyn, where Mr. de Blasio made the announcement, which had been widely anticipated. The mayor-elect referred to an old headline calling Mr. Bratton “America’s Top Cop” before allowing him to take the microphone. “Welcome back,” Mr. de Blasio said.

“You would bring that magazine cover up,” Mr. Bratton replied.

It was a moment of glory light years from the day Mr. Bratton began his policing career in Boston in 1970. He still speaks with the city’s accent.

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William J. Bratton as the head of the Los Angeles Police Department in 2009.CreditDavid McNew/Getty Images

He first came to New York to lead the transit police in 1990, going after fare-beaters and other low-level offenders in broad sweeps that also caught more serious criminals.

The approach was derived from the “broken windows” theory of policing, which holds that by cracking down in troubled neighborhoods on small but highly visible crimes of disorder such as turnstile jumping and prostitution, more serious crimes fall as well. It is a theory that Mr. Bratton has long espoused. Some researchers over the years since have questioned the efficacy of the theory, but Mr. de Blasio said he was a firm believer that it worked.

From the transit police he moved on to lead the Boston Police Department briefly before returning to New York City as commissioner in 1994. It was at a time when murders were around 2,000 a year, when squeegee-men were a fixture on the streets, and drug-dealers and prostitutes flourished in the open. It was also a time of widespread corruption in the department.

In his first weeks back in New York, Mr. Bratton made headlines by personally ejecting a panhandler from a subway car. Soon after, he went to Harlem’s 30th Precinct — which would later be called the “Dirty Thirty” — and, with television cameras rolling, confiscated the shields of officers who had been accused of corruption.

He gave more authority to precinct commanders and used a computerized crime-tracking system known as CompStat to provide detailed feedback on both crimes and the police response. Crime fell, even in the most crime-ridden precincts, such as the 75th Precinct in East New York, Brooklyn.

As arrests shot up, so did the number of complaints against the department — complaints that received scant attention in a city overwhelmed by crime. Mr. Bratton’s national profile began to rise.

Criminologists have long debated how much of New York’s crime decline can be attributed to the changes he put into place, and they have struggled to explain the strong downward trends across the country through the 1990s. “If we can’t explain that it in Toledo, we can’t explain it in New York either,” said Franklin E. Zimring, a criminologist at Berkeley Law School who has written on New York City’s crime decline. But, he added, “A lot of the long term success of policing in New York is something that is properly credited” to Mr. Bratton.

As Mr. Bratton became known as the crime-fighter who calmed America’s toughest city, his persona began to grate on Mayor Giuliani, who watched his own role in the story of New York’s declining crime seemingly eclipsed. Mr. Bratton appeared to have his eyes on becoming mayor himself one day (he writes of his strong poll numbers in his memoir, “Turnaround,” published in 1998) exacerbating tensions. Behind-the-scenes disputes between the Police Department and City Hall were routine.

A plan for a police parade on Mr. Bratton’s birthday was scuttled by Mr. Giuliani, who forced Mr. Bratton out in 1996.

On Thursday, Mr. Bratton acknowledged “differences” between himself and Mr. Giuliani and allowed that “some of those differences were created by me.”

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Mr. Bratton in 1995 with Mayor Rudolph W. Giuliani of New York, who appointed him commissioner in 1994.CreditJames Estrin for The New York Times

(For his part, Mr. Giuliani recently had warm words for Mr. Bratton, describing him at a Thanksgiving Day event as an “exceptional law enforcement officer.”)

After leaving the Police Department in 1996, Mr. Bratton took a number of security positions, including as president and chief operating officer at Carco Group, a fraud investigating company on Long Island.

In 2002, Mr. Bratton sought and won the top job at the Los Angeles Police Department, a shrinking and distrusted force that had entered a federal consent decree the year before, largely because of a corruption scandal that involved reports of abuse of suspects, evidence tampering and perjury. He reached out to the department’s critics early in his tenure.

By the time Mr. Bratton was reappointed five years later, relations between minorities and the police had improved, leaders there said. “He helped transform the L.A.P.D.’s relationship with the community it serves while bringing crime down to historic lows,” Antonio R. Villaraigosa, the mayor during Mr. Bratton’s tenure, said in a statement.

But police stops in Los Angeles doubled under Mr. Bratton, according to a 2009 analysis by the Kennedy School of Government at Harvard. Even with a federal consent decree, racial disparities persisted during that time. For those who trace the New York Police Department’s stop-and-frisk tactics, and the ensuing troubles, back to the policing strategies Mr. Bratton put into place in the 1990s, it is a sign that he had softened his image over the years, but not his approach.

“Given his record in L.A., the question then becomes, if this is the mayor who is going to stop the stop-and-frisk era, why is he going to bring in the person who increased stops in Los Angeles?” said Norman Siegel, who led the New York Civil Liberties Union from 1985 to 2000.

In recent years, Mr. Bratton traveled regularly to speak as a kind of management consultant for law enforcement officials worldwide.

After riots in London in 2011, David Cameron, the British prime minister, publicly entertained bringing in Mr. Bratton to lead the police there. He returned to New York in recent years as the chairman of Kroll, a corporate security and investigations company, before leaving that position to start up his own consulting business. In the last year, he began building a social media site for police officers.

He is married to Rikki Klieman, a lawyer, and has one adult son, David Bratton, by a previous marriage. At the announcement, Ms. Klieman sat with Chirlane McCray, Mr. de Blasio’s wife and confidant on policy.

“Over the years, he’s come to see the importance of dealing forthrightly with issues of race and police-community relations,” said Jeremy Travis, who worked for Mr. Bratton at the Police Department in 1994 and is now president of John Jay College of Criminal Justice. “He is very open to learning from his own experience.”

Joseph Goldstein and William K. Rashbaum contributed reporting.

A version of this article appears in print on , Section A, Page 1 of the New York edition with the headline: Bratton to Lead New York Police For Second Time. Order Reprints | Today’s Paper | Subscribe